It is known to use laser diodes for the purpose of generating light in optical data transmission systems. In this case, the optical output power of a laser diode is determined by a driver circuit, which supplies a bias current to the laser diode, said bias current being modulated in a manner dependent on the data signal to be transmitted.
DE 100 65 838 A1 discloses an electronic driver circuit for directly modulated semiconductor lasers. Said document describes how a constant signal, which is intended to be used to transport an item of information, is superposed by means of so-called peaking. This peaking influences the rising and/or falling edges of the current through the semiconductor laser. Peaking is caused by a driver circuit and makes it possible for the current through the semiconductor laser and thus the emission of light to be switched on and off more rapidly. This makes it possible for the semiconductor laser to transmit a high data rate.
FIG. 5 (taken from DE 100 65 838 A1) illustrates peaking: FIG. 5 diagrammatically shows how a voltage Vpk is superposed on the constant signal of the data transmission VDat. These two quantities yield the current IVCSEL flowing through the VCSEL laser. The voltage pulses VDat and Vpk and also the current pulses IVCSEL are plotted against time t. FIG. 3 diagrammatically shows the light radiation LICHTHL of a laser diode in dependence on the current flow IHL through the laser. In this case, the solid line in the diagram IHL relates to a data signal which has not been modulated by peaking and the dashed lines 1 and 2 indicate the behavior of the current through the laser if the signal has been modulated by peaking of varying magnitude. It can be seen in FIG. 3 that, as a result of peaking, the switch-on and switch-off edges of the light radiation LICHTHL become steeper and the laser light can be switched on and off more rapidly.
One disadvantage of the previously known circuit resides in the fact that peaking provides a negative current pulse, which lowers the forward voltage through the semiconductor radiation source to such an extent that switch-on delays may occur when the semiconductor radiation source is next switched on (that is to say in the event of the next positive current pulse). The switch-on delays depend on the lowering of the forward voltage across the semiconductor radiation source and correspond to the discharge of the diffusion capacitance. Exact matching of the negative current pulse to be used, for the purpose of improving the switch-off edge, proves to be technically impracticable since certain variations in the electrical and optical properties prevail in the components of the circuit.